Auto merge of #30707 - tsion:mir-text, r=nikomatsakis

r? @nikomatsakis

Textual MIR can be dumped for a particular `fn` with `#![rustc_mir(pretty = "filename.mir")]`. Below is an example of the text output.

```rust
struct Point {
    x: i32,
    y: i32,
}

fn example() -> Point {
    let mut e = Point { x: 1, y: 2 };

    let num = 5;
    let plus_num = |x: i32| x + num;

    e.y = plus_num(e.x);
    e
}
```

```rust
fn() -> Point {
    let mut var0: Point; // e
    let var1: i32; // num
    let var2: [closure@test.rs:84:20: 84:36 num:&i32]; // plus_num
    let mut tmp0: ();
    let mut tmp1: &i32;
    let mut tmp2: ();
    let mut tmp3: i32;
    let mut tmp4: &[closure@test.rs:84:20: 84:36 num:&i32];
    let mut tmp5: i32;
    let mut tmp6: Point;

    bb0: {
        var0 = Point { x: 1, y: 2 };
        var1 = 5;
        tmp1 = &var1;
        var2 = [closure@test.rs:84:20: 84:36] { num: tmp1 };
        tmp4 = &var2;
        tmp5 = var0.0;
        tmp3 = tmp4(tmp5) -> [return: bb3, unwind: bb4];
    }

    bb1: {
        return;
    }

    bb2: {
        diverge;
    }

    bb3: {
        drop var0.1;
        var0.1 = tmp3;
        drop tmp2;
        drop var2;
        drop var0;
        tmp6 = var0;
        return = tmp6;
        drop tmp6;
        goto -> bb1;
    }

    bb4: {
        drop var2;
        goto -> bb5;
    }

    bb5: {
        drop var0;
        goto -> bb2;
    }
}
```

```rust
fn(arg0: &[closure@test.rs:84:20: 84:36 num:&i32], arg1: i32) -> i32 {
    let var0: i32; // x
    let mut tmp0: ();
    let mut tmp1: i32;
    let mut tmp2: i32;

    bb0: {
        var0 = arg1;
        tmp1 = var0;
        tmp2 = (*(*arg0).0);
        return = Add(tmp1, tmp2);
        goto -> bb1;
    }

    bb1: {
        return;
    }

    bb2: {
        diverge;
    }
}
```